The present invention relates to an air conditioner including a motor-driven compressor operated with power supplied from a DC power source.
A conventional air conditioner for vehicles including a motor-driven compressor operated with power supplied from a DC power source is shown in FIG. 14. The air conditioner includes output unit 11 for DC/AC-converting a DC current for supply an AC current to motor-driven compressor 14, and capacitor 7 for smoothing the DC current of rectangular waveform. While breaker 3 being closed, the capacitor 7 is charged by battery 1 via fuse 2, breaker 3, and reverse-connection-protecting diode 4 through a charge resistor 5 for preventing a charge inrush current. The reverse-connection-protecting diode 4 protects the circuit by cutting off the current if the battery 1 is connected in reverse polarity.
Control unit 10, upon receiving a command to operate the motor-driven compressor 14 from air-conditioner controller 12, detects the charge of capacitor 7 and closes relay 6, for example, when the voltage of capacitor 7 detected by voltage detector 8 reaches a predetermined level. Then, the control unit 10 instructs the output unit 11 to drive the motor-driven compressor 14. The control unit 10 is supplied with power from 12V power source 13. Switching power supply 9 converts the voltage of battery 1 to supply the power to the output unit 11 and voltage detector 8. Control unit 10, upon receiving a command to stop the motor-driven compressor 14 from the air-conditioner controller 12, stops the output of the output unit 11 and opens the relay 6.
The control unit 10 is always in operation since receiving a power from 12V power source 13. Accordingly, the unit is always possible to communicate with controller 12, receive a signal from a sensor for air conditioning, and check the circuit with a circuit-checking device.
FIG. 15 is a circuit diagram of another conventional air conditioner. The control unit 10 receives a power from switching power supply 9, and receives no power from 12V power source 13. The conditioner has a simple structure in which a connection with 12V power source 13 is not needed, and a common ground can be used for the control unit 10 and other circuits. Constant current charging circuit 15 supplies a constant current obtained through subtracting a base-emitter voltage of a transistor from a zener voltage of a zener diode and dividing it by a resistance of an emitter resistor. For example, in cases that the zener voltage is 17V, that the base-emitter voltage 2V, and that the resistance is 50 ohm, the constant current is 0.3A.
If capacitor 7 is not charged for the case of being out of order, the voltage of capacitor 7 is 0V, and the supply voltage of switching power supply 9 connected in parallel with capacitor 7 is also 0V. In that case, switching power supply 9 is not in operation, and the control unit 10 receiving a power from switching power supply 9 is not in operation, either. Accordingly, control unit 10 cannot check the circuit, communicate with air controller 12, or receive the signal from the sensor before capacitor 7 is charged. The conventional conditioner in FIG. 14 involves no problems because the control unit 10 is always in operation. The conditioner in FIG. 15 has a similar problem even in case charge resistor 5 is provided in place of constant-current charging circuit 15.
While motor-driven compressor 14 is not in operation, capacitor 7 is preferably discharged for energy saving and operation time shortening in order to enhance the lifetime of the conditioner. However, if capacitor 7 is discharged, the control unit 10 does not operate as described above. In the conditioner shown in FIG. 14, the capacitor 7 is discharged with opening breaker 3, but there arises no problem because the control unit 10 is always in operation.
If capacitor 7 is discharged while motor-driven compressor 14 is not in operation, the capacitor 7 needs to be charged before start operating the motor-driven compressor 14, thus requiring time for motor-driven compressor 14 to start operating. This problem is common to the conditioner in FIG. 14.
In an air conditioner having a control unit receiving a power from a switching power supply, a supplied voltage for the control unit is reduced to a possible lowest voltage for the unit to operate. This arrangement provides the conditioner with less electromagnetic wave noise of the switching power supply, with a quick check and repair, and with having a motor-driven compressor start quickly. The air conditioner includes: a power application device connected in series to a direct-current (DC) power source; a switch device connected in parallel with the power application device; a capacitor charged by the DC power source via the power application device; a voltage detector for detecting a voltage of the capacitor; an output unit for driving a motor-driven compressor for air conditioning, the output unit receiving a power from the DC power source via the switch device; a control unit for controlling the output unit; a power supply connected in parallel with the capacitor, for supplying a power to the control unit, the power supply receiving a power from the DC power source; and a voltage lowering device for, when the motor-driven compressor does not operate, setting the voltage of the capacitor to a voltage which is lower than a voltage of the DC power source and which allows the control unit to operate. The voltage lowering device being controlled by the controller.